An actuator is a mechanical device that is powered by a certain source of energy, such as electric current, pressure and chemical energy, and can transform that energy into motion. According to the energy source utilized for actuation, responsive polymeric materials generally can be divided into three classes: electro-active polymers; light- or heat-responsive elastomers; and pH- or solvent-responsive gels.
Technologically, an actuator with controlled and directed motions upon application of a stimulus is particularly attractive because such a device could mimic organisms, and has numerous applications ranging from sensors, switches, and artificial muscles to nano/micro electromechanical systems. In a specific area, the actuation and power-generation systems that can harvest ambient energy from water gradients have recently attracted a great deal of attention, especially in the development of “dry” or liquid-free polymer actuators based on conducting polymers (CP) such as polypyrroles (PPy), polythiophenes, and polyanilines (PANI) as well as their composite systems. Thus, high hydrophilicity of the doped state, as well as the stress-strain generated from movement of water molecules directed by humidity variation, are being exploited.
For example, the doped PPy film containing perchlorate counter-ions would undergo rapid bending upon asymmetrical water-vapor sorption, and would crawl on a wet filter paper; culminating in the designs of a soft motor capable of directly transducing chemical potential of water sorption into a continuous circular motion, and origami (folded PPy film) actuators. Furthermore, the doped PPy film would contract in air under an applied voltage, thus generating Joule heating to desorb water molecules and providing an electromechanical control of the device.
In an alternative design in which semi-solid polyelectrolye had been used, a polymer composite film based on polypyrrole-polyolborate (PPy-POB) was shown to spontaneously and reversibly capture and release the ambient water vapor to induce film expansion and contraction, resulting in rapid and continuous locomotion on a wet surface. The PPy-POB machine was strong and powerful enough to lift objects 380 times heavier than itself, and transport cargo 10 times heavier than itself.
Another version of dry CP-actuators based on a polythiophene, viz. commercially available poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT:PSS), has been fabricated, wherein a nontraditional bilayer design with a collective capillarity feature to create the effect of asymmetric water diffusion. This PEDOT:PSS composite material is comprised of doped polyaniline (PANI) nanotubes chemically synthesized in-situ and embedded in a polycarbonate membrane, with one end of the nanotubes attached to a subsequently surface-deposited PANI layer. The so-called “nanotubes embedded membrane” (NEM) showed water diffusion behavior quite similar to those observed for biological ion channels and pumps, and displayed excellent moisture-propelled oscillatory motion, and artificial-muscle capability.
From the standpoint of dielectric materials, innovative approaches to humidity-driven actuation take advantage of the stress-strain generated from the orientational change in liquid-crystalline polymers (LCP) and networks (LCN) containing moieties (e.g. OH and COOH) that are sensitive to polar solvent vapors. As lyotropic or thermotropic LCP, cellulose derivatives, such as partially hydroxypropylated cellulose (HPC) and partially modified cellulose stearoyl ester (CSE) may have networks with solid state properties similar to LCE and LCN. Their solution-cast films are hygroscopic and have been shown to be promising materials in the development of humidity-powered, soft motors.
Building on a hygroscopic and mechanically robust liquid crystal network (LCN) polymer containing COOH groups that are capable of reversible hydrogen-bonding and become hydrophilic after alkaline treatment, another family of humidity actuators has been created. These new humidity-driven actuators may be in monolithic form or in bilayer configuration with a uniaxially oriented polyamide-6 substrate, and having large responses to humidity change as manifested in bending, folding, and curling motions. Bilayer actuators based on alternating layer-by-layer deposition of poly(cation)/poly(anion) films on hydrophobic polymer substrates and engineered to power devices capable of unidirectional and humidity-controllable locomotion on a ratchet track have also been described.
While the above-mentioned examples for humidity-driven actuators have illustrated several innovative bilayer designs and clever utilization of responsive polymeric and nanocomposite systems, it appears that no simple, wholly covalent polymer in monolithic form with hygromorphic and motile properties has been reported. Accordingly there is a need for new polymers and copolymers with hygromorphic and motile properties, as well as new methods for making them.